The present invention relates to differential pressure responsive indicators and more particularly to indicators which are temperature sensitive so as to prevent operation at low temperatures. The present invention further relates to such indicators which provide an internal reset feature.
In hydraulic systems wherein fluids such as oils or the like are passed through a filter, it is desirable to provide an external mechanism for indicating when the filter element has become clogged and requires replacement. Since the pressure drop across a filter increases in proportion to the accumulation of dirt therein, a suitable indication may be obtained by utilizing a differential pressure device set to be actuated when the pressure drop in the filter reaches a predetermined value. There are several known prior art devices which provide external means for indicating the clogging of the filter element and which compensate for low temperature operation, so that a false clogging signal is not indicated at low fluid temperatures.
One such design is shown in U.S. Pat. No. 2,942,572, to David Pall, issued June 28, 1960. In this device, a first magnetic element is arranged to attract a second magnetic element as long as the two elements are separated by less than a predetermined distance. A biasing mechanism propels the second magnetic element to an indicating position whenever the predetermined distance is exceeded. The first magnetic element is movable reciprocally with a piston responsive to changes in pressure, and is normally biased towards the second magnetic element by a predetermined force. The second magnetic element is also movable reciprocally with a piston. While the second magnetic element is retained towards the first magnetic element by magnetic attraction when close enough to it, it is normally biased in a direction away from the first magnetic element. The biasing force is capable of overcoming the force of magnetic attraction whenever the first and second magnetic elements are separated by the predetermined distance. The magnitude of the force of the magnetic attraction, relative to the spring biasing force, determines the predetermined pressure differential at which the device is actuated. In order to prevent actuation at low temperatures, a bimetallic strip is positioned to prevent motion of the second magnetic element when the temperature is below a predetermined value. The bimetallic strip includes two arcuate strip portions which are joined by a weld. The bimetallic strip contracts inwardly at low temperatures so that the inner strip extends over a flange, thus preventing actuation of the pressure indicator. Thus, the bimetallic strips are subjected to severe stresses at low temperatures. This leads to the eventual fatiguing of the bimetallic elements and failure of the pressure indicator. In addition, this device is complex and expensive to make.
Cole, in U.S. Pat. No. 3,117,550, issued Jan. 14, 1964, owned by the Assignee of the present application, discloses a temperature sensitive differential pressure indicating device which includes an elastically unstable element. The elastically unstable element consists of a spherically or conically shaped bimetallic disk which s held and sealed along its periphery so that the resultant force acting on the disk from the pressure differential will be in the direction tending to cause the disk to flatten out. When a predetermined pressure and temperature combination is reached, however, the disk will suddenly invert with considerable force and will stay inverted until manually reset or automatically reset when the actuation pressure is released. This device is also complex, requiring precise calibration of the disk to respond to the pressure and temperature combination.
Juhasz, in U.S. Pat. No. 3,812,816, discloses a differential pressure indicator with a thermally sensitive element. The device has a pressure sensitive mechanism and an indicator operatively associated with it. The pressure sensitive mechanism moves under the influence of a differential pressure generated in a fluid handling system to allow a suitable indication when a certain pressure differential exists over the parts of the system. The device further has a locking mechanism which insures that the indication will remain until the causes of pressure differential are corrected. The device also contains a temperature sensitive element in the form of a bimetallic coil strip which, at one end, is fastened to a piston body and, at the other end, is fastened to a head shank and a head, such as to rotatably drive the head. As the fluid temperature increases, the head is moved in a counterclockwise rotation by the expanding bimetallic strip until notches or cam surfaces are moved into a position behind the locking balls. The locking balls are then free to withdraw from partial engagement with a slot to enable the piston to freely move in response to any fluid differential pressure, so long as the fluid temperature is maintained at a preselected level. This design is also complex, requiring precision machining of the parts in order to form the locking mechanism.
Other devices, such as that disclosed in U.S. Pat. No. 3,974,795, use bimetallic elements and try to prevent the movement of a piston when the piston is subjected to the high pressure differentials due to cold fluid. This eventually leads to overstressing the parts with the eventual failure of the bimetallic elements.
All of the aforesaid prior art devices are complex. Many require expensive components, such as bimetallic elements. In all of the prior art devices, the thermal element is isolated from the working fluid.
It is also sometimes desirable to provide means for resetting of an indicating device internally when the filter has been replaced. Nonetheless, it is desired to prevent resetting of the device externally, so as to provide some degree of assurance, prior to resetting, that the filter has in fact been cleaned or replaced when the indicator is reset.
There are some existing devices which attempt to accomplish these results together, but unfortunately, they have functional limitations which result in either premature actuation or no actuation at all, depending on the conditions which are present.
What is needed, therefore, is a differential pressure responsive indicating device which combines the feature of a temperature sensing device, to prevent operation at low temperature, with an internal reset feature. Such a device should be reliable so as to prevent inadvertent operation when no clogging in fact exists in the filter since the reset is internal. Nonetheless, such a device should accurately indicate an actual clogging condition so as to prevent damage to other components, which might otherwise result from the clogging of the filter.